Health officials in Illinois and Iowa are investigating a link between McDonald’s salads and a rash of illnesses caused by a parasite.

About one quarter of 90 people sickened since mid-May in Illinois reported eating a McDonald’s salad in the days before falling ill, while an additional 15 cases were reported in Iowa.

The Iowa Department of Public Health said the affected residents who were later confirmed to be suffering from cyclospora infections ate McDonald’s salads in late June or early July. Officials in both states said that McDonald’s is cooperating with their investigators as well as the Centers for Disease Control and Prevention, and the Food and Drug Administration.